For a film carrier-type wiring board having a three-layer structure comprising, for example, IC chips for driving a liquid crystal connected to a glass substrate for liquid crystal panel formation, folding slits have hitherto been provided from the viewpoint of reducing the frame area of a liquid crystal panel for notebook computers and the like. This film carrier-type wiring board is constructed so that an outlet terminal is connected to the terminal part of the glass substrate constituting the liquid crystal panel and is folded at its slit part and an input terminal is located on the backside of the liquid crystal panel. This film carrier-type wiring board having a three-layer structure with folding slits formed therein is generally produced and supplied in a tape form provided with a number of film carriers. In the above film carrier-type wiring board, however, the thickness of the insulating layer is so small that the provision of folding slits lowers the rigidity of the tape, leading to a problem that the wiring pattern is likely to be broken due to strain caused, for example, during the transfer of the tape. In order to prevent such breakage, for example, Japanese Patent Laid-Open No. 351953/2001 proposes a method in which one slit is formed in the form of a plurality of divided slits to prevent a lowering in rigidity.
Recently, plasma displays (PDPs) which are quite different from liquid crystal panels (LCDs) as described above have drawn attention as visual display units. Unlike liquid crystal panels, in plasma displays, individual elements emit lights, and, thus, a higher brightness than that in liquid crystal panels can be achieved to realize high quality display. Further, in the case of plasma displays, increasing the size of the display is possible, and, thus, plasma displays can be used, for example, as visual display units for large-screen TVs.
In order to drive plasma displays, however, a higher voltage than required for liquid crystal panels should be applied. The film carrier-type wiring board for driving such plasma displays has a larger size than the size of the film carrier-type wiring board for LCD driving, and slits provided for permitting the film carrier-type wiring board, for driving the plasma display, to be used in a folded state should be elongated slits, for example, having a size of 1 mm in width and 50 to 100 mm in length. The formation of such long slits in an insulating film results in lowered rigidity of the film carrier in its slit formed part. Further, it should be noted that mass production of such large slits by a punching press at a high speed with good accuracy is very difficult. Accordingly, also in the film carrier-type wiring board for driving plasma displays, as with the wiring board for LCDs, one slit is formed in the form of a plurality of divided slits by punching.
In driving such PDP, in general, a voltage of not less than 60 V should be applied. This necessitates the adoption of a construction different from that in semiconductor devices for LCD driving. In particular, for film carriers for PDPs, the applied voltage is so high that generation of heat involved in voltage application poses a problem. Therefore, preferably, the film carrier for PDPs has flame retardant properties.
In general, the film carrier comprises a wiring pattern formed of an electrically conductive metal such as copper provided on an insulating film such as polyimide, and the wiring pattern except for its terminal part is covered with a solder resist layer. In this film carrier, the solder resist layer has the lowest heat resistance, and, in order that the film carrier is flame-retardant, the solder resist layer should be flame-retardant.
The solder resist layer is to be selected such that the solder resist layer has good affinity for the insulating film such as polyimide and the wiring pattern and, for example, in the case where folding slits are provided, a flexible solder resist ink is used for solder resist layer formation. There are many solder resist inks which are flexible and have affinity or the like for the wiring pattern or the insulating film. In liquid crystal panels (LCDs), however, since the applied voltage is not so high, rendering the solder resist layer flame-retardant has not been fully studied yet. A general method for rendering a resin flame-retardant is to incorporate a phosphorus compound or the like in the resin. Flame retardants such as phosphorus compounds used for imparting flame retardancy to the resin are finely dispersed in the resin, but on the other hand, they are not unified with the resin. Therefore, upon bleeding of the flame retardant, in some cases, the flame retardant solder resist layer is likely to be separated.
Further, the width of the insulating film is narrow at its part between divided folding slits, and, at the time of slit formation by punching, the insulating film (for example, polyimide film) in its part between slits undergoes strains and, consequently, is somewhat deformed (that is the insulating film in its part between slits is not flat), often making it difficult to smoothly coat the flame retardant solder resist ink onto this uneven part. When coating defects occurs in the flame retardant solder resist ink coated onto the part between such slits, lifting of flame retardant solder resist layer occurs in its part, resulting in abnormal appearance.
The solder resist layer is provided for protecting the wiring pattern, and the solder resist ink is coated with the greatest possible care so as to avoid the inclusion of foams in the solder resist layer. In this case, however, even when air bubbles contained in the solder resist ink could be substantially completely removed, very fine foams are sometimes included depending, for example, upon the contact angle of a squeegee and the state of screen. The included foams become pinholes having a large diameter (formed by foams having a diameter of not less than 1 mm). When pinholes are disadvantageously formed within the divided folding slits, deterioration in strength of adhesion to the insulating film is microscopically significant. In the inside of the divided slits, foams are accidentally formed. Therefore, even when all the air bubbles contained in the solder resist ink are removed, in some cases, fine air bubbles are caught up in the ink in the coating step. The inclusion of air bubbles in the ink and the occurrence of pinholes having a large diameter at the time of coating cannot be completely prevented by only the control of the solder resist ink.
It has been found that, in particular, when foams are included in the divided slits in the backside flame retardant solder resist layer provided on the backside of the insulating film, pinholes are likely to be formed in the flame retardant solder resist layer within the divided slits, particularly in the backside flame retardant solder resist layer in its part near the inner wall of the slits. Further, in particular, the flame retardant solder resist layer in its part between the divided slits on the wiring pattern formed face is likely to be lifted. The reason for lifting from the insulating film in its part between the divided slits has not been elucidated yet. However, possible reasons for lifting of the flame retardant solder resist layer attributable to a failure of adhesion or a failure of coating starts from a part near the divided folding slits, particularly between slits, include that the bleed of the flame retardant contained in the backside flame retardant solder resist layer is likely to concentrate on this part, that, in addition, the width of the insulating film in its part between the divided slits is very narrow and is not more than 5 mm, that, in the formation of divided slits by press punching, the insulating film such as polyimide undergoes strains and, consequently, in many cases, the surface of the insulating film in its part between the divided slits become uneven and the affinity of the flame retardant solder resist ink for the uneven part is likely to be lowered, and that, as compared with LCDs, the area of the individual divided slits is large.
In plasma displays of which the demand is being rapidly expanded, the voltage applied to the film carrier-type wiring board on which electronic components for driving the plasma display are mounted is high, and, thus, the development of a method for preventing the separation of the flame retardant solder resist layer susceptible to heat generated by the high applied voltage is urgently in need.